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The time rate of flow of light. It is radiant flux in the form of electromagnetic waves which affects the eye or, more strictly, the time rate of flow of radiant energy evaluated according to its capacity to produce visual sensation. The visible spectrum is ordinarily considered to extend from 380 to 760 nanometers in wavelength; therefore, luminous flux is radiant flux in that region of the electromagnetic spectrum. The unit of measure of luminous flux is the lumen. See Photometry
a photometric quantity. Luminous flux is radiant flux—that is, the power of the optical radiation —evaluated with respect to its capacity to produce visual sensation. More precisely, luminous flux is radiant flux evaluated according to its action on a selective optical detector whose spectral sensitivity is defined as a function of the spectral luminous efficiency V(λ), where λ is the wavelength of the light in a vacuum. The unit of luminous flux is the lumen. The relation between the luminous flux Фv and the radiant flux Фe is given by the equation
Here, Km is the maximum value of the spectral luminous efficacy; Km ≈ 680 lumens per watt at a wavelength of 555 nanometers.